1. Field of the Invention
The present invention relates to a linear motion rolling guide unit, which is applied to machine tools, industrial robots, precision machines and testing equipment and in which a slider comprising a casing and end caps is slidably mounted on a track rail through rolling elements.
2. Description of the Prior Art
The linear motion rolling guide unit generally has a slider mounted slidable on a track rail through rolling elements, the slider comprising a casing and end caps. To describe in more detail, the linear motion rolling guide unit comprises: a track rail having raceway grooves formed on both longitudinal sidewall surfaces thereof; a casing mounted slidable on the track rail and having raceway grooves formed therein that face the raceway grooves of the track rail; a pair of end caps attached to the longitudinal ends of the casing; rolling elements rolling between the opposing raceway grooves; end seals attached to the outer end surfaces of the end caps; under seals provided to the underside of the casing on both sides; and a top seal provided to the underside of the casing which faces the top surface of the track rail.
In the linear motion rolling guide unit, foreign matters such as dirt, dust and chips, once they adhere to the raceway grooves of the track rail and slider, block the smooth running of the rolling elements as the slider slides on the track rail, and will eventually damage the raceway grooves shortening the life of the product. To prevent this, the linear motion rolling guide unit has end seals, under seal and top seal as sealing means between the slider and the track rail.
Of the sealing means, the end seals effectively remove foreign matters adhering to the top surface of the track rail. When foreign substances collect in bolt holes in the track rail, it is difficult to remove them completely by the end seals. Foreign matters that have moved past the end seals are scattered inside the slider and may enter into the raceway grooves where the rolling elements are running. To prevent the foreign matters that have passed the end seals unhindered from entering into the raceway grooves, a top seal is provided.
The raceway grooves of the track rail and the slider are supplied with grease. When grease adheres to the upper surface of the track rail, foreign matter sticks to the upper surface of the track rail and becomes difficult to remove by the end seals alone. The linear motion rolling guide unit has the top seal to prevent grease in the raceway grooves from leaking into the top surface of the track rail. The top seal has two functions of preventing foreign matters from entering the raceway grooves and of preventing grease from leaking into the top surface of the track rail.
A known conventional top seal consists, for example, of a flat elongate plate and a single rubber lip secured to the edge of the plate and extending longitudinally. The lip is inclined outwardly toward the raceway grooves of the casing, with the end of the lip pressed against the top surface of the track rail by its own elastic force (Japan Patent Utility Model Publication No. 35945/1987, for instance).
Because of its outwardly inclined lip, the conventional top seal can prevent grease from leaking into the top surface of the track rail and also keep foreign matters from entering into the raceway grooves. The sealing performance of the top seal, however, is not sufficient because the lip has a single structure. For example, when a large foreign substance strikes the lip, it pushes the lip open and enters into the raceway grooves. Further, because it is made of an elastic material and its end is formed thin, the lip may be damaged by foreign matters. Possible countermeasures may include increasing the number of lips in the top seal to increase the sealing area and to make the passage of foreign matters difficult by the increased number of lips. This, however, results in an increase in the sliding resistance, making it difficult for the slider to follow a high-speed sliding motion.
Another example of the conventional top seal is a seal plate that extends longitudinally of the track rail and has an almost rectangular lateral cross section, and which is directly secured to the underside of the slider that opposes the upper surface of the track rail in such a way that the underside of the top seal does not contact the upper surface of the track rail, leaving a small gap between them. This top seal may be formed of a metal, plastics or synthetic rubber (Japan Patent Utility Model Laid-Open No. 82330/1989, for example).
Although the top seal has the advantage of zero sliding resistance because it (i.e., seal plate) is not in contact with the track rail, its sealing performance cannot be said to be sufficient in terms of grease leakage. Because there is a small gap between the underside of the seal plate and the upper surface of the track rail, fine foreign matters can pass through the gap into the raceway grooves. Another problem is that when the seal plate is made of a metal, foreign matters may get seized in the gap increasing the sliding resistance of the slider enormously.